JP2818253B2 - Dissolution method of iron-based scrap contaminated with radioactive materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention safely dissolves iron-based scrap contaminated with radioactive materials, such as dismantled nuclear power plants, without the radioactive materials diffusing out of the system. It is about the method.

[Prior Art] In order to dissolve iron-based scrap contaminated with radioactive materials, conventionally, dusting equipment is provided, or melting and refining is performed in an induction furnace or arc furnace having a closed structure. However, there is no known dissolution condition for preventing the radioactive substance from diffusing other than slag and metal, and for example, there is no description in the following report. DSHarvey: Melting of co
ntaminates steel scrap from decommissioning (Proce
eding of 1989 International Conference on the Deco
mmissioning of Nuclear Installations. 24-27 Oct., 1
989, Brussels p.422-430), M. Sappok and G. Lukac
s: Melting of Radioactive Metal Scrap from Nuclear
Installations (ibid., P431-442).

[Problems to be Solved by the Invention] In the conventional method, radioactive substances, particularly cesium and manganese, contained in the raw iron-based scrap are often scattered as fumes and dust. For this reason, exhaust gas treatment equipment such as dust collection and its operation become large-scale and complicated.
This is a problem that occurs because the temperature of molten iron and slag becomes high, even temporarily and locally, and the slag composition is not sufficiently studied. An object of the present invention is to solve such a problem, and concentrates radioactive materials on slag or molten iron, thereby reducing the rate of transfer to fumes and dust. It provides a dissolution method.

[Means for Solving the Problems] The gist of the present invention is to (1) dissolve iron-based scrap contaminated with radioactive materials, set the iron bath temperature to 1400 ° C or less, and dissolve the carbon content of iron. Is 3% or more, and the slag composition is 0.3 to 1.0 in weight ratio of calcium oxide to silicon oxide in the slag, and is a method for dissolving iron-based scrap contaminated with radioactive materials. (2) The amount of fluoride added to the slag is 1.5% or more and less than 18% when the basicity of the slag is more than 0.5 and 1.0 or less as fluorine, and 1.5% or more when the basicity of the slag is 0.5 or less. The method for dissolving an iron-based scrap contaminated with a radioactive substance according to the above (1), which is less than 12%.

 The details of the present invention will be described below together with the operation.

[Operation] The iron-based scrap as the raw material in the present invention is mainly steel scrap, but scrap obtained by dismantling used nuclear power generation equipment and the like, and includes pressure vessels, various pipes, equipment, buildings, and the like. , Foundations and other structures. Therefore, radioactive cobalt, manganese, cesium, strontium, and the like generated with the use of a nuclear reactor are included as radioactive substances contaminating scrap. It is preferable that the scrap is classified and sorted according to the radioactivity level, and the radioactivity level is further reduced by decontamination or the like.

The scrap may be charged in its entirety from the beginning, or may be charged by dissolving the seed water in advance and adding it.

The melting furnace used in the present invention has a sealable structure to prevent the diffusion of radioactive materials, scrap,
It is equipped with an inlet for auxiliary materials such as coke and a solvent, an outlet for molten iron and slag, and an exhaust gas suction and dust collection mechanism.
An electric melting furnace such as an arc melting furnace, a plasma melting furnace, or an induction melting furnace is preferable in that the operation is easily controlled.
All radioactive substances contained in the dust pass through the dust collection system,
During operation, the pressure inside the melting furnace is kept lower than the atmospheric pressure so as not to escape from the system.

As a result of investigating the conditions for radioactive substances, especially cesium to become fumes and escaping into dust, and remaining in the slag, the molten iron has a high carbon composition and the temperature at the time of melting is kept low. And it was found that it was essential to keep the basicity of the slag low. That is, in order to keep more than 80% of cesium in slag,
It is necessary to keep the temperature at 00 ° C. or lower, and preferably at 1350 ° C. or lower if possible. Since the temperature of the slag is lower than the temperature of the molten iron in the induction melting furnace, it is more advantageous in retaining cesium in the slag. Since the temperature of the slag is higher than the temperature of the molten iron in the arc melting furnace, it is preferable to operate at a lower temperature.

In order to operate at a low temperature, the molten iron needs to have a high carbon composition. In order to maintain stable operation during melting and maintain fluidity at the time of tapping, the concentration of carbon in the molten iron must be 3% or more, and a higher concentration is more advantageous. In order to enable a lower temperature operation, the carbon concentration is preferably 4% or more. The fact that the molten iron has a high carbon composition is advantageous for keeping the degree of slag oxidation low, which has the effect of suppressing the transfer of cesium and manganese to dust.

The basicity of slag is generally defined by the weight ratio of calcium oxide to silicon oxide in the slag. In order to prevent radioactive substances, especially cesium from remaining in the slag and fumes and escaping into dust,
It must be less than 1.0. Further, as long as the slag is in a molten state, the lower the basicity, the more advantageous the suppression of escape.
However, if the basicity is too low, the melting point of the slag increases, and the slag does not become a molten slag at an iron bath temperature of 1400 ° C. or less. Since silicon oxide accumulates in the slag as the scrap melting progresses, it is important to keep the basicity within a predetermined range throughout the melting. If the basicity of the slag is low, the slag solidified and cooled becomes glassy, and has the advantage of being physically and chemically stable. When an induction melting furnace is used, the low basicity of the slag is also advantageous for suppressing the wear of the furnace refractory.

In the present invention, by adding an appropriate amount of fluoride to the slag, it has succeeded in suppressing the wear of the furnace refractory while maintaining good slag fluidity even in low-temperature operation.

Fluoride has the effect of lowering the melting point of oxide slag and increasing the fluidity at low temperatures, and it is known that the greater the amount, the greater the effect. However, it has been found that when the fluorine content increases in a low basicity slag as in the present invention, the amount of dust increases and the basicity also increases with time. At the same time, the fluorine content in the exhaust gas also increases, and the refractories are easily eroded. Therefore, it is not preferable to add too much fluoride.

As a result of various studies on the type and amount of fluoride added to the slag, the present inventors have found out the type and optimum amount of fluoride to be added. That is, calcium fluoride and sodium fluoride are suitable as the kind of the added fluoride from the viewpoint of thermal and chemical stability, and natural fluorite having a higher purity is most suitable.

It is effective to add 1.5% or more of fluorine, and 4%
As described above, the effect becomes remarkable. As the amount of addition increases, the amount of dust also increases. When the basicity of the slag is more than 0.5 and less than 1.0, it is 18% or more, and when the basicity of the slag is less than 0.5, it is more than 12%. Become. Therefore, the amount of fluoride added is 1.5% when the basicity is more than 0.5 and 1.0 or less in terms of fluorine.
Above, less than 18%, and 1.5% when the basicity is 0.5 or less
As mentioned above, the useful range is less than 12%.

[Example] In a 0.5 ton low-frequency induction melting furnace equipped with a closed structure, equipped with exhaust gas suction treatment equipment, and lined with siliceous amorphous carbide, dismantled reactor structure and 350 kg of low-level radioactive scrap Was charged together with 16 kg of coke and melted while maintaining the inside of the furnace at a reduced pressure with respect to the atmospheric pressure. As a flux, 1 kg of quicklime and 0.1 kg of fluorite were added before meltdown. Melted and refined at a temperature of 1250-1330 ° C, 1310
At ℃, the molten metal was discharged from the furnace bottom together with the slag and cast into a mold. The obtained iron component is C 3.8%, Si 0.62%, Mn 0.35%, slag is CaO 17.8%, SiO ₂ 55.2%, T.Fe 2.5%, CaF ₂ 3.3%, C
s 0.05%. The amount of dust collected from the electric dust collector and the thickener was very small at 0.5 kg. The dust component was SiO ₂ 13.4%, CaO 3.3%, and the balance was iron with no Cs detected.

The solidified slag is vitreous, its radioactivity is 0.00
013 μCi / g and the radioactivity of dust was 0.00001 μCi / g or less.

As a comparative example, the same raw material scrap was melted together with 5 kg of coke. The flux added 6 kg of quicklime before burnt off. Melting was carried out at a temperature of 1450 to 1565 ° C, and immediately after melting, tapping was carried out together with slag from the furnace bottom at 1540 ° C and cast into a mold. The obtained iron components are C 0.5%, Si 0.30%, Mn 0.28
%, Slag is CaO 36.8%, SiO ₂ 31.5%, T.Fe 4.5%, Cs
It was less than 0.01%. The amount of dust collected from the electric dust collector and the thickener was 3.5 kg. The component of dust is SiO ₂
The 9.6%, CaO 11.2% residue was mainly iron and Cs 0.03%.

The solidified slag is crystalline, and the radioactivity of the slag is 0.
00009 μCi / g. The radioactivity of dust is 0.00014μCi / g
And 10 times or more the value of the above-mentioned embodiment of the present invention, which is much higher.

[Effect of the Invention] According to the present invention, iron-based scrap contaminated with radioactive material can be dissolved without imposing an excessive burden on environmental measures such as dust, exhaust gas, and wastewater treatment, and the radioactive material is converted into iron and stable glassy slag. Can be migrated. Thereby, there is an advantage that iron-based scrap from a dismantled nuclear reactor can be stored in a small volume with no danger of diffusion. When the radioactivity level of iron is low, it can be reused as a raw material for cast iron or steel.

Claims (2)

(57) [Claims] (1) In dissolving iron-based scrap contaminated with radioactive materials, the iron bath temperature is set to 1400 ° C. or less, the carbon content of the dissolved iron is set to 3% or more, and the slag composition is reduced to silicon oxide in the slag. A method for dissolving iron-based scrap contaminated with radioactive materials, wherein the weight ratio of calcium oxide is 0.3 or more and 1.0 or less. 2. When the weight ratio of calcium oxide to silicon oxide in the slag is more than 0.5 and 1.0 or less in the slag, the amount of fluoride added to the slag is 1.5% or more and less than 18%.
The method for dissolving iron-based scrap contaminated with radioactive materials according to claim 1, wherein when the ratio is 0.5 or less, the ratio is 1.5% or more and less than 12%.